This invention relates to a charging system for a battery-powered drive such as used, for example, for a fork lift truck.
One proposal for a system for a battery-powered vehicle has been described and claimed in our pending UK Patent Application No. 9809802.3.
With reference to the circuit shown in FIG. 1, an AC cage type induction motor is shown at 10 and is supplied with power by means of a DC powered drive circuit in the form of a variable frequency inverter 11. The inverter 11 is of any convenient type and can itself be controlled in order to provide for speed and directional control of the motor. The power supply to the inverter is derived from a DC-DC converter which includes a capacitor C which is charged from the vehicle battery by controlling the operation of a solid-state switch QI. The switch, when closed, causes current to flow from the battery into an inductor L and when the switch is opened, the flux collapse in the inductor induces a high voltage which is added to that of the battery to charge the capacitor by way of the diode DI. The diode prevents discharge of the capacitor when the switch is reclosed.
The switch, which in this case is shown as an IGBT, is subject to mark/space ratio control and the voltage Vc at the capacitor terminals is related to the battery voltage Vb by the equation:
Vc=Vb.Tp[/Toff
where Tp is the sum of the on and off times of the switch and Toff is the off time of the switch.
It will be noted that as Toff tends to zero, Vc tends to infinity. In practice boost ratios of 5-6 to one are attainable.
FIG. 2 shows a modification to the circuit of FIG. 1 in order to pass energy from the capacitor to the battery. This is achieved using the solid-state switch Q2 which is connected in parallel with the diode DI and a further diode D2 is connected in parallel with the switch Q1. By controlling the operation of the switch Q2 energy can be passed to the battery, thus providing regenerative braking of the motor.
FIG. 3 shows a modified circuit which produces an inverted output voltage. A further inductor L2 and a further capacitor C2 are introduced into the circuit. When switch QI is closed the flux builds up in the inductor LI and when the switch is opened capacitor C2 is charged as in the example of FIG. 1. When the switch is reclosed the charge on capacitor C2 is transferred to capacitor CI by way of the second inductor.
The circuit can operate in the reverse direction by operating the switch Q3, for the purpose of transferring energy back to the battery and providing regenerative braking.
In the circuit shown in FIG. 4 an upward voltage conversation is effected in two stages. LI, CI, Q1 and the diode of Q2 form a well-known boost converter circuit. The switching of QI will produce a voltage in excess of that of BTI across CI as a function of the switching mark to space ratio. Whilst it is theoretically possible to utilize this stage to achieve the full step up range (in the case shown 80 to 560 volts) in practice a step up exceeding 5:1 becomes inefficient and a second stage consisting of Q3, Q4, L2 and C2 is provided. Q3 and Q4 are switched on a 50:50 basis with the result that a voltage across C2 is produced equal in magnitude to that across CI, as shown in the diagram. The resulting 560-volt bus feeds the motor drive inverter.
Under energy recovery conditions, the charge on CI is transferred to the battery via L1, Q2 and the diode of Q1 acting in the well known buck converter configuration, where energy from a high voltage source can be transferred to a low-voltage load.
The action of C2, Q3 and 04 corresponds to the above-described actions, ensuring that the potentials across C1 and C2 remain constant and equal.
Using the circuits described it is possible to power volume produced high voltage motor inverter units from the low voltage batteries fitted on battery-powered vehicles, thereby enabling such vehicles as fork lift trucks, to be manufactured more cheaply.
According to some embodiments of the invention, a drive system for a battery-powered vehicle including an AC or DC motor includes a rechargeable battery and a voltage boost circuit for enabling the motor to be provided with power at a voltage greater than that of the battery and for enabling the speed of the motor to be controlled. The boost circuit includes contactors simultaneously movable to lie either at a first position when the battery is discharging to the motor, or at a second position when the battery is being recharged. The boost circuit further includes an inductor which includes a secondary winding which is used only during recharge of the battery.
According to further embodiments of the invention, a battery-powered vehicle includes an AC or DC motor. The vehicle further includes a drive system include a rechargeable battery and a voltage boost circuit for enabling the motor to be provided with power at a voltage greater than that of the battery and for enabling the speed of the motor to be controlled. The boost circuit includes contactors simultaneously movable to lie either at a first position when the battery is discharging to the motor, or at a second position when the battery is being recharged. The boost circuit also includes an inductor which includes a secondary winding which is useful only during recharge of the battery.